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Trends of common birds in Europe, 2009 update, computation procedure and data quality control in details

Data and computation
Map of European countries contributing with their data.


The data came from 22 countries: Austria, Belgium, Bulgaria, Czech Republic, Denmark, Estonia, Finland, France, Germany, Hungary, Ireland, Italy, Latvia, Netherlands, Norway, Poland, Portugal, Slovakia, Spain, Sweden, Switzerland, United Kingdom.
Slovakia is a new country in data set and has provided data for the time period 2005-2007. This year, data from the Netherlands cover an enlarged time period (1984-2007 instead of from 1990 onwards as in earlier years). Data from Belgium come from two regional monitoring schemes in Wallonia and Brussels only and cover the period until 2005. Data from former West Germany were treated separately from the data from former East Germany (for computation procedure only). For Estonia, non-updated indices were used (the period until 2006 only). Data for France, Latvia and Sweden come from different schemes within a country, covering different time periods. The data from these schemes were combined first to produce the national indices these countries covering the combined periods. This was done using an adapted version of TRIM in a similar way as the aggregation of the data of all countries in PECBMS.
Countries within the same group (region) used for calculation regional and European indices (as indicated also in the table below) are in the same colour in the map.

Country/regionRegion(group of countries)First yearLast year
AustriaWE19982007
Belgium-Brussels1)WE19922005
Belgium-Wallonia1)WE19902005
BulgariaSEE20052007
Czech Republic CEE19822007
DenmarkWE19762007
EstoniaCEE19832006
FinlandNE19752007
France2)SE19892007
Germany East3)CEE19912007
Germany West3)WE19892007
HungaryCEE19992007
ItalySE20002007
Latvia4)CEE19952007
NetherlandsWE19842007
Norway NE19952007
PolandCEE20002007
PortugalSE20042007
Republic of IrelandWE19982007
SlovakiaCEE20052007
SpainSE19962007
Sweden5)NE19752007
SwitzerlandWE19992007
United KingdomWE19662007


WE - West Europe
NE - North Europe
SE - South Europe
SEE - Southeast Europe
CEE - Central & East Europe
First year - first year of data time series in a country/region
Last year - last year of data time series in a country/region
Time series for individual species from national schemes are shorter in certain cases.
1) Data for Belgium were combined from Wallonia and Brussels and cover the period until 2005.
2) Data in France come from two schemes, old (1989-2001) and new one (2001-2007). Data from both schemes were combined.
3) Data for Germany were combined from schemes in former East and West Germany.
4) Data for Latvia come from three different schemes, two old ones (differ in their regional coverage, and cover the periods 1995-2006 and 2003-2006, respectively) and a new one (2005-2007). Data from all schemes were combined.
5) Data for Sweden come from two schemes, old (1975-2007) and new one (1998-2007). Data from both schemes were combined.

In hierarchical process of the European species indices computation (see the figure below), the national indices are firstly combined into regional ones. This allow to estimate missing values in countries with shorter time series in TRIM (a process analogical to production of indices at national level) firstly within these groups of countries (regions) where we expect similar changes in bird trends and so to avoid e.g. estimating missing values in South European country using data from the North. Then, yearly totals for missing years for countries within a group were estimated.
Weighting factor for each country and species was calculated as population size (geometric mean of population minimum and maximum provided by Birds in Europe 2 (BirdLife International 2004)) divided by estimated (model) scheme year total for the same years.
Combined year totals and their standard errors for whole group were then calculated using weighting factors.

Overview of computation steps incl. groups of countries:

Data quality control
National data on species trends were checked using the following criteria.

1. European species indices were computed if data are available from countries which altogether host at least 50 % of ´PECBMS European´ population of a species. ´PECBMS Europe´ means the set of countries included in our definition of Europe for assessment of abundant and widespread species. This includes countries which already contribute actively by data provision or are supposed to provide data by 2010: Austria, Belgium, Bulgaria, Czech Republic, Denmark, Estonia, Finland, France, Germany, Hungary, Italy, Latvia, Lithuania, Netherlands, Norway, Poland, Portugal, Republic of Ireland, Romania, Slovakia, Spain, Sweden, Switzerland, United Kingdom.
However, some parts of countries (states) listed above were not used and their population not considered in the assesment. These are: Faroe Islands and Greenland, Svalbard, Azores, Madeira, Canary Islands, Gibraltar.

2. The reliability of the national data were inspected using the following criteria (see below) and all suspicious results were examined in detail and either excluded from the data or validated. National species indices were excluded from the computation in case the suspicious results were not justified as reflecting real population changes. This procedure also included consultations with coordinators of national monitoring schemes.

National species data were subjected to closer examination when:

  • Slope (Multiplicative) < 0.6
  • Slope (Multiplicative) > 1.5
  • Slope (Multiplicative) standard errors > 0.5
  • Percentage of scheme time totals of the species > 95% of national population size of the species in Birds in Europe 2 (BirdLife International 2004)
  • Ratio of national population size to scheme time totals > maximum of species population size in Birds in Europe 2 (BirdLife International 2004)
  • Number of zero counts < 1
  • Number of missing counts < 1
  • Index value < 0.5
  • Index value > 1000
  • Scheme time totals < 1
  • Scheme time totals > 1000000
  • More than one year with index = 100 and SE = 0 present in the results
The data quality checks were done by an automation system developed by Statistics Netherlands.

3. As a part of more precise and careful data quality control, all national indices - species by species - were also checked for their interannual consistency (comparison of previous and new version of trends and indices) this year and all suspicious and inexplicable inconsistencies in indices were examined in details. The same control for consistency was also carried out for supranational (regional and European) indices and trends. Generally, all European and regional indices and trends were found well consistent with previous version. In case some inconsistencies had occurred, those were always satisfactorily explained by enlarged data set (new countries contributed with the data).

4. European and regional species were checked for their use for production of indicators too. If a species index is classified as ´Uncertain´* AND index value is >>>> 200 % or <<<< 5 %, then the species index and data quality were examined in details. These criteria were considered as indicative, final decision taken (i.e. species to be potentially excluded from an indicator) considers also whether a species was used already in previous versions of the indicators, whether better data can be expected in near future and whether index fluctuation is believed to be caused either by poor data or by reasons not linked directly to habitat quality. This rather conservative approach is used in order to prevent bigger influence of subjective decision.
The same assessment has been done for all versions of indicators produced.

*1) Trend classification

The multiplicative overall slope estimate (trend value) in TRIM is converted into one of the following categories. The category depends on the overall slope as well as its 95% confidence interval (= slope +/- 1.96 times the standard error of the slope).

  • Strong increase - increase significantly more than 5% per year (5% would mean a doubling in abundance within 15 years). Criterion: lower limit of confidence interval > 1.05.

  • Moderate increase - significant increase, but not significantly more than 5% per year. Criterion: 1.00 < lower limit of confidence interval < 1.05.

  • Stable - no significant increase or decline, and most probable trends are less than 5% per year. Criterion: confidence interval encloses 1.00 but lower limit > 0.95 and upper limit < 1.05.

  • Uncertain - no significant increase or decline, and unlikely trends are less than 5% per year. Criterion: confidence interval encloses 1.00 but lower limit < 0.95 or upper limit > 1.05.

  • Moderate decline - significant decline, but not significantly more than 5% per year. Criterion: 0.95 < upper limit of confidence interval < 1.00.

  • Steep decline - decline significantly more than 5% per year (5% would mean a halving in abundance within 15 years). Criterion: upper limit of confidence interval < 0.95.

    Petr Voříšek and Jana Škorpilová

    13 November 2009

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